1. Field of the Invention
This invention relates to liquid crystal display systems, and particularly to methods and means for detecting and displaying faults in such systems.
The invention is especially directed to methods and means for displaying improper functioning of a segement electrode (or electrode segment), a group of segments, back electrode, feed lines or leads, etc., in standard liquid crystal display (LCD) systems in which separately controllable electrode segments are energized in a static manner, or by multiplexing, to exhibit numerals, characters, or symbols.
Liquid crystal displays have assumed greater and greater importance in the field of electro-optical indicators as their degree of development has increasingly progressed. A typical example of their application is in a numerical read-out in which each numeral is formed by energizing appropriate segments of a 7-segment component. The individual 7-segment component or several together may form an LCD module or cell.
The basic construction of a liquid crystal display is known. Generally, two glass plates facing one another are coated on their opposing surfaces with thin transparent or translucent conductive layers and sandwich a nematic liquid crystal between them. Typically, one of the opposing layers is divided into the separate conductive electrode segments and the other layer forms a back electrode. When a voltage is connected from the thin back electrode layer to one or more of the segments formed from the other electrode layer, the resulting electrical field forms zones of different refractive indices or different dispersions in the liquid crystal so that one zone can be distinguished from another.
Depending upon the structure of the segments, the liquid crystal cell can be used for displaying numerals, letters, or other informative symbols. The display can be made reflective by using incident light, transmissive by using transmitted light, or transreflective using both transmitted and incident light, depending upon different arrangements of reflectors.
Such liquid crystal displays may be considered passive as distinguished from active displays such as, for example, those using light-emitting diodes (LED's), because basically they do not produce their own light, but rather merely modulate ambient light.
One of the many important advantages of a liquid crystal display is its very low current consumption. However, this very low current consumption makes it practically impossible to check the operation of a liquid crystal display. This contrasts with active displays, such as LED displays, where checking of the display is simple because the individual current flows can simply be measured and compared retroactively with prior information. An errror signal can thus be derived, if necessary. On the other hand, a similar segment check in passive displays, such as liquid crystal displays, does not produce any usable result. This difficulty arises because of the very numerous low segment currents which fluctuate relatively sharply.
2. Description of Related Art
Redundant displays have been proposed to increase the reliability of LCD's. Such a display appears in DE-OS No. 24 02 749. Another certainty-enhancing proposal involves more extensive segmentation of the individual character symbols. This appears in DE-OS No. 24 59 488. However, these schemes raise production and circuit costs considerably.
To counter this unsatisfactory state of liquid crystal displays, German patent document DE-OS No. 27 43 907 proposes that the front plates carry additional electrodes, known as check electrodes, which are electrically connected with each of the segment electrodes, i.e., the "endangered" electrodes. During failure or incorrect control of one the segment electrode, the system shows another character from the character reservoir of the display. At the same time, the back plate with the back electrode contains another back electrode, i.e., a "check back electrode", located only opposite the check electrodes. However, this requires additionally connecting practically all the so-called endangered segment electrodes, that is, at least five segment electrodes in a 7-segment numerical display, to a check electrode. It also requires the back plate of the display to contain an additional check back-electrodes which cooperate exclusively with the check electrodes. Furthermore, it requires that the carrier plates accommodate five check electrodes and a check back electrode as well as a checking circuit in addition to the conventional 7-segment electrodes and back electrode. This constitutes a substantial expenditure because of the large quantity of separately distributed electrical connections that are necessary. It also creates a large source of errors. Moreover, this system is costly and fails to furnish any possibility for checking conventional LCD modules.
The disclosures in the German patent document DE-OS No. 30 26 848 and DE-OS No. 29 51 584 attempts to obtain error interpretations via various segment electrodes and back electrode arrangements. In DE-OS No. 30 26 848, each segment electrode is connected to a driver circuit for conventional control via a first conductor path, and at the same time, a current loop is formed for each segment control via the connection by means of a second conductor path. A test capacitor in each current loop is charged and discharged during the control of the respective segment when the segment is activated by multiplexing. A resistance in the discharge current loop, which is identical for all test segment connections, produces a measurable voltage drop which can be triggered by means of the discharge current. However, such devices are complicated and expensive.
In DE-OS No. 29 51 584, the control signal to the segment electrode from the control conductor is transferred to a second separate conductor path via an electrically anisotropic resistance layer and is supplied to monitoring electronics as a check signal. In the two monitoring circuits, an error display must finally be initiated in an error reporting device.
Standard LCD's cannot be monitored, or, if so, only with considerable difficulty by checking the or control current or the control output. Accordingly, the methods for meeting the requirements set forth for checking are directed toward providing a high Hamming distance (point matrix) or providing redundancy by means of double segments or by means of segment electrodes and back electrode arrangements in multiplex systems. The redundancy responds to interference or disturbance by preventing segment combinations which can be incorrectly interpreted.
When error sources and their effects on LCD system are analyzed, the actual error sources can be localized in the area of the display element or in the area of the control circuit. As a rule, errors in the display module, such as leaky medium cells caused by aging, exceeding, or dropping below, permissible operating temperature ranges, glass breakage, defective polarizes, and the like, affect all segments or the entire display modular component. To this extend, such a defective phenomenon can be immediately seen and recognized.
Unobvious display errors are, as such, not detectable by the viewer. As a rule, they occur in the control area when individual segments fail or are incorrectly activated in the course of the display operation. An example of such failures are interruptions of signal lines or conductors, defective contacts at the display module, or defective conductor paths. Faults that subject display segment configurations to incorrect interpretation can occur continuously or intermittently. An improperly energized segment can change an 8 to 9 or a 0 without a detectable cause for a signal interruption to the segment. Therefore, an unconditional requirement exists for means which are able to detect and display incorrect indications that cannot be perceived by the eye. This is especially important for official approval of devices which are capable of being calibrated, such as taximeters, price indicators or gasoline products, remote transmission installations, and the like.